The invention relates, in general, to protocols for transmitting digital data and, in particular, to protocols for short haul digital systems such as local area networks.
It has been known for some time that digital data can be transmitted over serial and broadcast media. A problem continuously faced by the designer of digital data communication equipment is efficient utilization of the transmission and receiving equipment as well as efficient utilization of the medium or channel over which the data is to be transmitted and received. A number of approaches have been developed in the past, most of which suffer from one or more drawbacks. One of the earlier well-known digital data control systems is the Aloha System, originally developed for a packet radio application at the University of Hawaii and put into public use more than twenty years ago. The Aloha System, in its pure form, is based upon a broadcast transmission followed by a listening period for an acknowledge signal from the receiving station. If no acknowledge signal is received, the transmitting station then retransmits randomly until it receives an acknowledgement signal indicating that successful transmission has been achieved. The Aloha System, in its pure form, allows variable length data slots or frames to be transmitted. However, Aloha suffers from the drawback that, on average, its Aloha maximum efficiency is about 18%.
An improvement over the pure Aloha system is slotted Aloha, which fixes the periods for data transmission to a fixed time or a slot time, also known as a data slot. The system uses the same transmission followed by acknowledgement as the pure Aloha but, due to the use of the fixed length data slots, achieves, maximally, up to 36% efficiency in channel utilization. CSMA systems have been developed which are useful for relatively short length systems, where "a", which is the ratio of the signal propagation delay to the time duration between the beginning of frame or slot transmission and the termination of frame or slot transmission, is less than 0.5. In those systems, CSMA is attractive. In order to practice the CSMA protocol, each station sharing a broadcast or other medium "listens" to the medium and does not initiate a transmission unless its response to listening indicates that the channel is currently unoccupied by a transmission from any other station. Such systems, however, do not achieve high throughput, in part because the maximal dimension of the system is dictated by the propagation delay to frame length ratio. This does not provide for efficient channel utilization.
The CSMA/CD system provides an improved and more efficient protocol over that of the CSMA system because the CSMA system, upon hearing a collision occurring, backs off for a period of time determined by an exponential back-off algorithm which is executed in appropriate software or hardware logic.
A significant improvement over the prior systems involves a digital protocol wherein a number of nodes, or stations, may all be connected to a single broadcast medium, whether wired or wireless, or may be connected in a star configuration or other configurations. Each of the stations includes a nodal apparatus which has a storage which may include a memory for storing a conflict resolution queue and a transmission queue. The system is a slotted system in that periodically, and at regular intervals, one or more control minislot signals may be transmitted from a particular station followed by a data transmission mission in a data slot in response to conditions in the conflict resolution queue and in the transmission queue. Such a system achieves significantly improved utilization of the channel capacity, in some cases, approaching 1.00 of the channel capacity.
One of the drawbacks of such a distributed queue random access protocol system, which is disclosed in Xu, Wenxin, "Distributed Queuing Random Access Protocols for a Broadcast Channel," Illinois Institute of Technology, Chicago, Ill., Dec., 1990, and U.S. Pat. No. 5,390,181 lies in the fact that for certain systems, such as local area network systems which not only have bursty transmission, but have transmission wherein the amount of data to be transmitted may vary significantly from time to time. Thus, if the fixed length data slot used in the basic distributed queue random access protocol is employed, there may be some channel inefficiencies which result due to the data slot not being entirely filled by a particular data transmission, thus causing some wastage of channel capacity. Likewise, inefficiency may result because a frame longer than the data slot must be segmented and, of course, associated with its own respective control minislots which effectively add unneeded overhead. What is needed is a system which employs conflict resolution queues and transmission queues in combination with a flexible data slot assignment and control system to enhance further the inherent efficiencies in the distributed queue random access protocol system.